1. Field of the Invention
This invention relates to a projection type display apparatus such as a liquid crystal projector, and particularly to an anti-reflection coat for use in the color separating and combining optical system of a liquid crystal projector using a reflection type liquid crystal panel.
2. Related Background Art
In a liquid crystal projector using a reflection type liquid crystal display panel, anti-reflection film is necessary on a color separating and combining prism to improve the contrast of a color image.
White light from a lamp is separated into lights of three primary colors, i.e., red, green and blue (hereinafter referred to as R, G and B, respectively) by the color separating synthetic prism. Accordingly, it is conceived that anti-reflection film optimized for white light is formed, for example, on the end surface of the color separating and combining prism onto and from which the white light is incident and emerges or the end surface of the color separating and combining prism onto and from which light beams of three primary colors, R, G and B are incident and outgone.
FIG. 7 of the accompanying drawings shows the construction of such anti-reflection film, and FIG. 8 of the accompanying drawings shows the spectral characteristic of the anti-reflection film of FIG. 7.
The anti-reflection film shown in FIG. 7 is optimized for white light, and has high refractivity layers, intermediate refractivity layers and low refractivity layers alternately formed by the use of three materials. The construction of this film is specifically an improvement in a substrate-.lambda./4 (intermediate refractivity layer)-.lambda./2 (high refractivity layer)-.lambda./4 (low refractivity layer) air type anti-reflection film (.lambda. being the design wavelength), and the intermediate refractivity layer which is the first layer is constructed of equivalent film of the high refractivity layer and the intermediate refractivity layer, and the high refractivity layer which is the second layer is constructed of equivalent film of the high refractivity layer/the intermediate refractivity layer/the high refractivity layer.
However, the reflectivity of this anti-reflection film is of the order of 0.2% in the wavelength band of 400-700 nm, and even if this anti-reflection film is formed on the end surfaces of a color separating synthetic prism on and from which light beams of three primary colors, R, G and B, are incident and outgone, a quantity of unnecessary reflected light which cannot be neglected enters a screen and therefore, the contrast of a color image in a liquid crystal projector cannot be heightened.
Also, the liquid crystal projector according to the conventional art has suffered from the problems that because three materials are used, film deposition cannot be done if not in a special apparatus for thin film deposition, the control of the film thickness and refractive index of each layer is severe and the cost is high, and the film thickness becomes high and therefore the film deposition tact time becomes long and the cost becomes high.